1. Field of the Invention
The present invention relates to a configuration of a magnetic circuit for a speaker device.
2. Description of Related Art
Conventionally, there is known a speaker device including an external-magnet type magnet circuit having a yoke, a magnet and a plate, and a vibration system having a voice coil, a voice coil bobbin, a diaphragm, a cap and a frame. For example, in the speaker device, a magnetic circuit has a configuration as follows. The yoke has a pole piece formed into a cylindrical shape or a cylindrical column shape, and a flange part outwardly extending from a lower end portion of an outer peripheral wall of the pole piece. A ring-shaped magnet is arranged on the flange part, and a ring-shaped plate is arranged on the magnet. A magnetic gap is formed between the pole piece and the plate.
However, in the magnetic circuit having the configuration, since a thickness of the plate and a thickness (i.e., height) of the pole piece (also referred to as “center pole”) in a direction of the thickness of the plate are different, a magnetic field lacking of concentration of a magnetic flux is formed in the vicinity of a lower side of the magnetic gap. Thereby, the magnetic field formed on an upper side of the magnetic gap and the magnetic field formed on the lower side of the magnetic gap are set to be asymmetrical in an up-and-down direction with respect to a center line passing through a center in a direction of the thickness of the plate. By occurrence of the asymmetrical magnetic field, the voice coil, the voice coil bobbin and the diaphragm cannot be moved in the axis direction of the speaker device. Then, it becomes problematic that secondary harmonic distortion occurs and sound quality is deteriorated.
As a magnetic circuit for solving the problem, there is proposed a magnetic circuit for a speaker device having a T-shaped pole piece (see Japanese Utility model Application Laid-open under No. 60-22092 and Japanese Patent Application Laid-open under No. 56-128099, for example). In the magnetic circuit, the thickness of the plate and the thickness of the upper end part of the pole piece forming a T-shaped part are formed substantially same. Thereby, the magnetic field formed on the upper side of the magnetic gap and the magnetic field formed on the lower side thereof are set to be symmetrical in the up-and-down direction, which solves the above problem. In addition, an example of the magnetic circuit having the shape similar to the T-shaped pole piece is disclosed in Japanese Patent Applications Laid-open under No. 6-14393 and No. 6-54398.
There is known a magnetic circuit for a speaker device formed by providing an alloy magnet at an upper part of a pole piece in a yoke and arranging a resin magnet layer on a surface of the alloy magnet (see Japanese Patent Application Laid-open under No. 4-48899, for example). By this configuration, a magnetic circuit having a magnetic gap formed with high accuracy and high magnetic energy can be obtained.
The above-mentioned T-shaped pole piece is integrally formed by a cutting work of a work. However, many parts of the work have tone cut in order to form the T-shaped pole piece. Thereby, it becomes problematical that a manufacturing cost of the T-shaped pole piece increases and thereby a manufacturing cost of the magnetic circuit also increases. As a result, the magnetic circuit of the above-mentioned type is generally applied to a luxury model speaker device at high price.